Generation of THz-radiation by difference-frequency mixing at one- and two-photon resonances

In this work, difference-frequency generation is theoretically analysed for the three-level quantum system, having forbidden in dipole approximation electronic and optically active vibration transitions from the ground state. Excitation of coherent phonon polaritons wave in the terahertz frequency range by an ultrashort optical pulse, being in the visible or near-infrared spectral regions, and injected pulse with frequency almost twice that of the pump is considered. Dynamics of terahertz radiation arising during excitation of both electronic and vibrational coherences at the two-photon and combined resonances in the ensemble of three-level quantum systems was evaluated.